1. Field of the Invention
The present invention relates to a power supply system in which voltage conversion is performed bidirectionally between either or both of two batteries and an output electric circuit.
2. Description of Related Art
A hybrid vehicle and an electric vehicle that employ a rotary electric machine as a driving source are provided with a power supply system including a battery, which is a direct-current power supply, and a voltage transducer for stepping up a battery voltage and stepping down a regenerative electric power caused by the rotary electric machine. As the voltage transducer, there has been known a buck-boost converter as illustrated in FIG. 28. The buck-boost converter steps up/down a voltage by ON/OFF operations of two switching elements S1, S2.
Further, as a voltage transducer having an expanded function of the buck-boost converter, Japanese Patent Application Publication No. 2012-070514 (JP 2012-070514 A), for example, describes a voltage transducer including four switching elements S1 to S4 and connected to two batteries B1, B2, as illustrated in FIG. 29. The voltage transducer can step up/down a voltage by ON/OFF operations of the four switching elements S1 to S4, and can switch the two batteries between series connection and parallel connection by changing ON-OFF patterns of the switching elements.
In the meantime, in a conventional voltage transducer, it is difficult to perform voltage conversion at the time of an ON fault (short-circuit fault) of a switching element. For example, in a buck-boost converter illustrated in FIG. 28, as generally known, a voltage is stepped up/down by an ON/OFF operation of a switching element S2 in synchronization with an ON/OFF operation of a switching element S1. Accordingly, when either one of the switching elements S1, S2 has an ON fault and is turned on continuously, a voltage cannot be stepped up/down in the above manner.
In the voltage transducer illustrated in FIG. 29, even if either of the switching elements S1 to S4 has an ON fault, a voltage can be stepped up/down theoretically by use of either of the batteries B1, B2. However, in this case, another problem is caused such that the stepping up/down of a voltage should be stopped from the viewpoint of safety. For example, at the time of an ON fault of the switching element S3, the battery B1 is connected in series with the battery B2, so that an output voltage is a voltage sum of the battery B1 and the battery B2. This voltage might exceed withstanding voltages of system main relays SMR1, SMR2, and the like, so the voltage transducer illustrated in FIG. 29 eventually cannot help but stop the stepping up/down of a voltage at the time of an ON fault of the switching element.